Field of the Invention
The present invention relates to a pattern antenna and an antenna device including a pattern antenna.
Description of the Background Art
In recent years, many small-size devices with wireless communication functions have been developed. Demands for miniaturizing an antenna to be incorporated in such a small-size device are growing.
Conventionally, F-shaped pattern antennas are widely used as antennas to be incorporated in small-size devices. An F-shaped pattern antenna is configured by forming patterns on the surface of a printed circuit board such that an antenna element is F-shaped. This enables an antenna for high frequencies to be formed in a relatively small area on the printed circuit board.
Furthermore, techniques for improving antenna characteristics by changing the shape of an antenna element (pattern shape on the printed circuit board) in the F-shaped pattern antenna have been proposed (e.g., see Patent Literature 1 (JP 2009-194783A)).
However, with the above conventional techniques, it may be difficult to achieve an antenna having desired antenna characteristics. This will be described with reference to FIG. 10.
FIG. 10 is a diagram showing an example of a conventional F-shaped pattern antenna 900. As shown in FIG 10, the F-shaped pattern antenna 900 includes a substrate 91, a ground plane 92 formed with a pattern on the substrate 91, and an antenna element portion 93 connected to the ground plane 92. Also, as shown in FIG. 10, F-shaped pattern antenna 900 includes feed points 94 and 95.
When the wavelength of the carrier wave used by the F-shaped pattern antenna 900 is λ, adjusting the length L91 of the antenna element portion 93 shown in FIG. 10 to a length corresponding to approximately λ/4 achieves preferable antenna characteristics (frequency characteristics). Furthermore, when the F-shaped pattern antenna 900 is adjusted such that its input impedance matches 50Ω, adjusting the distance from the feed point 94 to the GND plane (the distance corresponding to the portion indicated by the arrow M1 in FIG. 10) and the position of the feed point 94 (the length L92 shown in FIG. 10) enables the capacitance component and the inductance component to be adjusted, thus allowing the input impedance to be closer to 50Ω.
The F-shaped pattern antenna 900 shown in FIG. 10 is configured to include the antenna element portion 93 extending in the vertical direction in FIG. 10, and the length L91 needs to be set to the length corresponding to approximately λ/4. This makes it difficult for the pattern antenna to be configured in smaller area while maintaining the antenna performance of the F-shaped pattern antenna 900.
In view of this, to configure a pattern antenna in smaller area while maintaining the length of the antenna element, it is conceivable to form the antenna element portion with bent portions (to make the antenna element portion meander line shaped) like the pattern antenna 900A shown in FIG. 11.
However, in the pattern antenna 900A shown in FIG. 11, space required for the short-circuiting portion 931A that extends toward the feed point 94A from the meander line shaped portion of the antenna element portion 93A that is positioned closest to the GND plane 92A is narrow. In other words, as shown in FIG. 11, adjustable area for the position of the short-circuiting portion 931A is limited, thus making it difficult to adjust the position of the short-circuiting portion 931A, achieve desired antenna characteristics, and perform appropriate impedance matching in the pattern antenna 900A.
While there is a strong demand for achieving a pattern antenna with excellent broadband characteristics, it is extremely difficult to achieve a small-sized pattern antenna with excellent broadband characteristics using the above-described conventional technique.
In view of the above problems, it is an object of the present invention to provide a pattern antenna, with excellent broadband antenna characteristics, that is formed in a small area.